Following the demonstration that lead salts can inhibit photosynthesis in isolated chloroplasts at concentrations as low as 50 uM, we plan to: A. begin a critical analysis of the inhibition of photosynthesis by lead salts with emphasis on locating the exact site of interference of the heavy metal with electron transport system. B. study the lead binding reaction at the molecular level. C. extend our previous work on lead inhibition in higher plants to unicellular algae. Using this less complex system we will ask: 1. to what extent the metal ion is taken up by the cell and eventually the chloroplast, 2. will the chloroplast level of lead or other metal be high enough to impair photosynthesis in vivo as it does in vitro. D. study competition of anions (such as phosphate), other cations, and chelating agents either natural or artificial with the metal binding reaction and the eventual inhibition of photosynthesis. E. apply our findings to the general study of the organization of photoreaction-II and the oxygen evolving mechanism of photosynthesis. The lead cation has been the only simple reversible inhibitor of photosyntem-II thus far reported. Additionlly we plan to extend these studies to the effects of cadmium, molybdenum, chromium, and other important environmental trace metals.